Method of focusing the horizontal and vertical components from an echelle grating

ABSTRACT

A method for focusing the horizontal and vertical components of energy reflected from an echelle grating which includes rotating the grating about a first axis substantially parallel to a prism face and rotating the grating about a second axis substantially normal to the first axis.

United States Patent Elliott 1 1 Apr. 25, 1972 [54] METHOD OF FOCUSINGTHE 2,671,128 3/1954 Zworykin et al. ..356/83 x HORIZONTAL AND VERTICALOTHER PUBLICATIONS COMPONENTS FROM AN ECHELLE GRATING William G.Elliott, Lincoln, Mass.

Spectra Metrics, Incorporated, Burlington, Mass.

Jan. 14, 1971 Inventor:

Assignee:

Filed:

App]. No.:

Related US. Application Data Division of Ser. No. 7l0,88l, Mar. 6, I968,abandoned, Continuation of Ser. No. 106,561, Jan. 14, 1971.

References Cited 01117151) STATES PATENTS 13/1953 Koppius ..356/83 xPRISM 11 INCIDENT ENERGY D ISPERSED ENERGY Mark: The Review ofScientific Instruments, Vol. 15, No. 2, February 1944, pages 28- 36Tolanksy: High Resolution Spectroscopy, Pitman Publishing Corporation,1947, pages 231- 237 Harrison: Journal of the Optical Society ofAmerica, Vol. 39, No.7, July 1949, pages 522- 528 Harrison et al:Journal of the Optical Society of America, Vol. 42, No. 10, Oct. 1952,pages 706- 712 Bausch & Lamb Catalog D-260, Echelle Spectrographs, pagesI- 24, received US. Patent Office Aug. 19, I955 Tarasov: Optics andSpectroscopy, Vol. 1 I, No. 5- 6, Nov.-Dec. 196i, pages 368 and 369Primary E,\'aminerRo nald L. Wibert Assistant Examiner-F. L. EvansAttorneyRichard P. Crowley and Richard L. Stevens [57] ABSTRACT A methodfor focusing the horizontal and vertical components of energy reflectedfrom an echelle grating which includes rotating the grating about afirst axis substantially parallel to a prism face and rotating thegrating about a second axis substantially normal to the first axis.

7 Claims, 4 Drawing Figures GRATING l2 UPRIGHT /-MOUNTING PLATE 19 x,ISHAFT l6 BASE l3 FATENTEDAPR 25 I972 SHEET 10F 2 UPRIGHT MOUNTINGJ/PLATE l9 ,GRATING MOUNTING PLATE GRATING l2 BASE l3 /SHAFT 15 SHAFT l6SHAFT l4 INCIDENT ENERGY PRISM u DISPERSED ENERGY F I G.

UPRIGHT MOUNTING PLATE I9 GRATING l2 SHAFT l4 BASE l3 INCIDE ENERG DISPERSED ENERGY F l G 2 METHOD OF FOCUSING THE HORIZONTAL AND VERTICALCOMPONENTS FROM AN ECHELLE GRATING PRIOR COPENIDING APPLICATIONBACKGROUND or THE INVENTION Spectrometers are devices employed formeasuring the spectral energy distribution impinging on its entranceaperture. In general, there are two types; the dispersive type whichcauses energy to be concentrated in space as a unique function ofwavelength, and the interferometer type which produces interferencepatterns distributed in space as a function of wavelength. Thedispersive type must be used in any situation where it is necessary toisolate a particular wavelength interval, i.e., in experiments whichdepend upon the energy per photon of impinging energy. Two methods areconventionally used to provide the dispersion; a prism or a grating.Operation of the former method depends upon the variation of photonvelocity as a function of the energy per photon hence it is highlydependent upon appropriate materials being available. The latter methodutilizes interference between wavelets reflected from various portionsof a ruled surface. Because constructive interference canoccur for anyintegral number of waves between adjacent grooves on the grating,spatial separation as a function of wavelength is not unique. Hence itis necessary to employ some form of order sorting method used inconjunction with the grating.

Because the mechanical variability in the groove spacing and angle isusually associated with the fabrication of such grating, it has not beenpossible to use such gratings under conditions in which the wavelengthbeing measured is in the visible region of the spectrum and where thenumber of wavelength differences between adjacent grooves is large. Anexception to this is the use of a so-called echelle grating for farinfrared spectroscopy where the groove dimensions are sufficiently largethat adequate tolerance can be obtained using machine tools in theirmanufacture. Recently, however, new techniques for controlling theruling of gratings have made it possible to produce gratings withadequate precision to be used in high orders in the visible andultraviolet region of the spectrum. Spectrometers employing suchgratings have been investigated and constructed and instruments areavailable which use a combination of two spectrometers in series, one ofwhich is used to select the order of the other. The majority of theseinstruments however, operate only under a small wavelength interval atany given setting. This is because conventional gratings capable ofyielding high resolution also produced an angular spread too large to becollected and focused conveniently.

A novel spectrometer employing an echelle grating has now been foundwhich is not subject to the deficiencies of the prior art.

SUMMARY OF THE INVENTION The novel spectrometer of the present inventionis com prised of the following essential components:

an entrance aperture at least a first collimating mirror a prism anechelled grating at least a first exit focal plane.

The grating is mounted in the spectrometer so that it rotates in twodirections: (1) around an axis parallel to the prism mounting axisparallel to the prism face, and (2) around an axis parallel to thegrating rulings and thus perpendicular to the first grating mountingshaft, thus providing means for adjusting the vertical and horizontalcomponents of the dispersed energy in the focal plane.

By means of the aforementioned grating rotational abilities, thehorizontal and vertical components of the dispersed energy in the exitfocal plane can be adjusted independently, thereby providing greaterresolution and order spacing than has heretofore been possible.

The prism is rotatable around a horizontal axis which passes through thecentral ray of the incident beam and parallel to the first surface ofthe prism, i.e., the face of which the incident beam falls. By pivotingthe prism, the angle between the incident energy and the prism face canbe adjusted without ap' pre'ciably altering the location of the incidentbeam on the prism face. Permitting only a single degree of rotationalfreedom in the prism ensures that the dispersion of the prism is notinadvertantly added to the dispersion of the grating. Preferably, theface of the prism most distant from the grating is nondispersive, whichobviates the necessity of moving the collimating mirrors.

While the configuration of the entrance aperture is not critical, it ispreferred that the length is five to ten times the width.

The superior resolution available with the novel spectrometer of thepresent invention can be illustrated with reference to the IronTriplet." The industry employs a rule of thumb indication ofacceptability of a spectrometer by referring to the so-called IronTriplet, i.e., a series oflines at 3020 A. An instrument is consideredsatisfactory if three lines can be distinguished on a spectrograph withlow power magnification. By means of the high degree of resolutionobtainable with the novel device of the present invention, spectrographsare obtainable which show not only the three lines of the "Iron Tripletbut actually five lines, and which can be viewed without magnification.

By proper choice of prism geometry and material combined with a echellegrating having an appropriate number of grooves and groove spacing, aresolving power of 250,000 can be readily obtained throughout thespectral region from 1200 A to 0.50 microns.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration in diagrammaticalform of a plan view of the novel spectrometer of the present inventionshowing the arrangement of the prism with respect to the grating;

FIG. 2 is an elevation view of the components of FIG. 1;

FIG. 3 is an illustration in diagrammatic form of the spectrometer ofthe present invention; and

FIG. 4 is an illustration in diagrammatic form of an optical system foruse with the spectrometer of the present invention for forming an imageof the grating surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsin FIGS. 1 and 2, there'is shown in diagrammatic form the relationshipof the prism to the echelle grating and the mounting and relative axesof rotation of said prism and grating. Incident energy from a source(not shown) impinges upon prism 11 which is mounted on a prism mountingplate 18 and which rotates on shaft 14. The incident energy passesthrough prism 11 and is dispersed along a first coordinate extending inthe direction of the length of the slit. The thus-dispersed incidentenergy from the prism 11 is incident on echelle grating 12 which ismounted on grating mounting plate 17 with shaft 16 parallel to therulings on said grating 12 and wherein said grating mounting plate isconnected to shaft 15 which is perpendicular to shaft 16 and parallel toprism shaft 14. Shaft 14 is fastened to upright mounting plate 19 whichis secured at a right angle to base 13. The reflected energy from theechelle grating 13 now passes back through prism 11 where it is furtherdispersed and directed toward an output collimator (not shown). Theatrangement of the prism and grating as shown in FIGS. 1 and 2 providesthe maximum dispersion by both the prism and the echelle grating whileat the same time permitting a reasonable size for both the prism andgrating and also maintaining a compact beam which enables the instrumentto be a relatively small size as compared with prior art commercialdevices.

As shown in FIGS. 1 and 2 the prism and grating are supported by avertical mounting plate which in turn is supported by a base plate. Thebearings for the shaft are mounted upright in the vertical mountingplate. in an alternative embodiment, the base plate may be mounted inthe instrument frame by a shaft which rotates about a vertical axispassing through the intersection of the central ray of the incident beamand the first surface of the prism.

The collimating mirrors employed in the present invention are eitherspherical or an off-axis parabola.

In FIG. 3 there is illustrated in diagrammatic form the completespectrometer of the present invention. Energy from source 25 is focusedby optics 26 on aperture slot 21. The energy passing through aperture 21is incident on collimating mirror 22 which directs iton to a prism 11which disperses the energy which is then incident on echelle grating 12.Prism 11 rotates about shaft 14 while grating 12 rotates about shaft 16and shaft (not shown). The dispersed energy reflected from the groovefaces in grating 12 is returned through prism 11 to collimating mirror23 from whence it is focused at the exit focal plane 24.

In the above drawings the orientation of the grating is selected toreturn the energy directly from the grating to the output collimator bypassing it through the prism for a second time. In an alternativeconfiguration the orientation of the grating is such that the energyreflected from the grating is directed to the collimator without asecond pass through the prism. This alternative embodiment is desirablein applications where stray energy introduced by scatter from theincident beam of energy by the first surface of the prism isundesirable.

The novel spectrometer of the present invention, by reason of thearrangement of the prism and the grating with respect to each other,provides an essentially square focal plane for at least one spectraloctave. This yields, simultaneously, higher resolution and a broaderspectral range with desirable properties in that only a small range ofangles with respect to the central ray of the focal point is produced.

The novel spectrometer of the present invention has a focal planeconfiguration which is particularly suited to the use of two dimensionalelectrooptical sensor methods. As examples of suitable sensors which maybe employed in conjunction with the novel spectrometer of the presentinvention mention may be made of detector arrays, image tubes, imageintensifiers, image convertors, spatial/temporal encoders and opticalcorrelators.

Conventional methods for quantitating the output of a spectrometereither utilize photographic film as an intermediate storage media withsubsequent quantitative densitometry of the film record or employ one ormore photoelectronic sensors to produce an electrical outputcorresponding to the particular wavelength incident upon that sensor.The configuration of the focal plane of conventional spectrometers doesnot permit the use of two-dimensional electrooptical devices such asimage tubes conveniently. A major advantage of the type of displayproduced by the novel echelle spectrometer of the present invention isthat the two-dimensional nature of the display combined with theresolution scaling introduced by successive orders permits the use oftwo dimensional electrooptical quantitative sensors. An additionaladvantage of this type of spectrometer of the present invention is thata relatively small range of angles of the beams is involved in formingthe focal plane image; hence it is relatively simple to recollect theenergy following the initial spectral focal plane and perform additionalreimaging. This latter advantage is particularly useful in situationswhere it is desirable to modify the spectral data prior toelectrooptical detection. In cases where it is known that a wide dynamicrange of spectral components exists, appropriate filtering can beintroduced at the first spectral focal plane to compensate for thedynamic range and avoid local saturation of the detector and equalizethe intensity of the spectral lines of interest so that a sensor havinglimited dynamic range, such as an image tube, can be employed.

An alternative embodiment is to introduce into the first spectral focalplane a spatial/temporal encoder and then collect the transmitted energyon a single detector. This technique provides a unique time sequencecode corresponding to each resolution element in the spectral focalplane. The corresponding intensity may be decoded from the output of thesingle detector by utilizing temporal autocorrelation. in the event thatonly a small portion of the spectral data is desired, multiple reimagingcan be utilized with a stationary transmission mask in the firstspectral focal plane followed by reimaging into a second spectral focalplane where the spatial/temporal encoder is located.

Referring now to FIG. 4, an example of the multiple reimaging is shown.The output energy from spectrometer 31 through spectral focal plane 30passes through condensing optics 32 to provide a first image 33 of thegrating face. The emitted beam then passes through reimaging optics 37to provide a second image 34 of the spectral focal plane and thence toreimaging optics 35 which provides a second image 36 of the gratingface. Optionally a spatial encoding disc or other type of encoder or animage tube may be employed at the spectral focal plane 36 or at asubsequent image of the spectral focal plane 34.

The multiple line, essentially square focal plane geometry provided bythe echelle spectrometer of the present invention also permits useage oftwo dimensional electrooptical devices such as image intensifiers andimage converters in addition to conventional image transducers such asvidicon or image orthicon. These devices may be used to enhance thesensitivity of thc system or to extend the spectral range of the imagetube.

Two dimensional incoherent optical filtering methods, i.e., an opticalcorrelator, may also be used with the type of display generated by thespectrometer of the present invention. Optical correlation utilizingmany spectral lines per element provides enhanced sensitivity overconventional techniques which employ only one spectral line per element.

The basic capability of the spectrometer to observe and detect a broadrange of the spectrum at high resolution is useful whether the energybeing analyzed is due to emission by a material or due to absorption bythe material from a known source. Particular instrument configurationsmay be suited by specific problems but the basic advantages of thisinstrument approach are the two dimensional nature of the focal planeand the resolution scaling introduced as various orders are observed.

A suitable echelle grating for use in the present invention is comprisedof 73.25 lines per millimeter, with a groove face blaze angle ofapproximately 63. A 30-6090 prism of calcium fluoride provides thenecessary order separation and broad spectral coverage. Focal lengthsand slit sizes are selected based on the particular instrumentconfiguration and the purpose for which the instrument is to beemployed.

This dispersing system may be combined for example, with 1 meter focallength optics to yield an instrument having reciprocal dispersion of l.4 angstroms per mm at a wavelength of approximately 5000 A or 0.7 a/mm.at 2500 A, and covering the spectral range from 1500 A to 6000 A on asingle 4 X 5 plate in the focal plane. Resolution is approximately 0.03A at 2500 A with a 50 micron slit width.

For use with'an image tube such as a vidicon, the focal length of theoutput collimator is preferably reduced to approximately mm. This yieldsa reciprocal dispersion of l 1.2 A/mm or a resolution of approximately0.5 A with a conventional vidicon.

A very compact system compatible with packaging for an electronics rackis obtained with 0.5 meter focal length optics. The reciprocaldispersion is comparable to that obtained with a conventional 3 meterinstrument (2.8 A per mm) yet it occupies less than a foot of verticalspace, and less than 2 feet of depth. Resolution is approximately 0.1angstroms at 5000 A (limited by optical abberations).

Unlike prior art devices, the spectrometer of the present invention iscapable of simultaneous coverage of a wavelength range exceeding afactor of 100 to l with an essentially constant ratio between spectralresolution and wavelength. It is also unique in that the range ofdispersion angles remain small; hence it permits a compact equipmentconfiguration. For example, in the near ultraviolet, results comparableto a meter prior art device are obtainable with a 1 meter device of thepresent invention.

Having described my invention, what I now claim is:

1. A method of focusing spectral energy distribution which comprises:

a. directing incident radiation from a source through an aperture;

b. dispersing said radiation by passing said radiation through a prismface;

0. receiving the dispersed radiation on an echelle grating;

d. reflecting said radiation as horizontal and vertical components; and

e. focusing the horizontal and vertical components of the radiation atan exit focal plane by:

i. rotating the grating about a first axis, said first axissubstantially parallel to the prism face; and

ii. rotating the grating about a second axis, said second axissubstantially normal to the first axis and parallel to the gratingrulings.

2. The method of claim 1 which includes receiving the radiation from theaperture on a collimating mirror and subsequently reflecting saidradiation prior to dispersing said radiation.

3. The method of claim 1 which includes dispersing the horizontal andvertical components from the grating and subsequently focusing saidhorizontal and vertical components at the exit focal plane.

4. The method of claim 3.which includesreceiving the horizontal andvertical components on a collimating mirror and subsequently focusingsaid components at the exit focal plane.

5. The method of claim 1 which includes forming a multiplicity of focalplanes.

6. The method of claim 1 which includes detecting the radiation in saidexit focal plane.

7. A method for focusing spectral energy distribution which comprises:

a. directing incident radiation from a source through an aperture;

b. receiving the radiation from the aperture on a collimating mirror andsubsequently reflecting said radiation;

c. impinging the radiation on a prism face;

d. dispersing said radiation by passing said radiation through theprism;

e. receiving the dispersed radiation from the prism on an' echellegrating; f. reflecting said radiation as horizontal and verticalcomponents; g. varying the horizontal and vertical components of theradiation at an exit focal plane by: l. rotating the grating about afirst axis, said first axis substantially parallel to the prism face;and 2. rotating the grating about a second axis, said second axissubstantially normal to the first axis and parallel to the gratingrulings; and passing the horizontal and vertical components from thegrating through the prism and subsequently focusing said horizontal andvertical components at the exit focal plane.

1. A method of focusing spectral energy distribution which comprises: a.directing incident radiation from a source through an aperture; b.dispersing said radiation by passing said radiation through a prismface; c. receiving the dispersed radiation on an echelle grating; d.reflecting said radiation as horizontal and vertical components; and e.focusing the horizontal and vertical components of the radiation at anexit focal plane by: i. rotating the grating about a first axis, saidfirst axis substantially parallel to the prism face; and ii. rotatingthe grating about a second axis, said second axis substantially normalto the first axis and parallel to the grating rulings.
 2. The method ofclaim 1 which includes receiving the radiation from the aperture on acollimating mirror and subsequently reflecting said radiation prior todispersing said radiation.
 2. rotating the grating about a second axis,said second axis substantially normal to the first axis and parallel tothe grating rulings; and h. passing the horizontal and verticalcomponents from the grating through the prism and subsequently focusingsaid horizontal and vertical components at the exit focal plane.
 3. Themethod of claim 1 which includes dispersing the horizontal and verticalcomponents from the grating and subsequently focusing said horizontaland vertical components at the exit focal plane.
 4. The method of claim3 which includes receiving the horizontal and vertical components on acollimating mirror and subsequently focusing said components at the exitfocal plane.
 5. The method of claim 1 which includes forming amultiplicity of focal planes.
 6. THe method of claim 1 which includesdetecting the radiation in said exit focal plane.
 7. A method forfocusing spectral energy distribution which comprises: a. directingincident radiation from a source through an aperture; b. receiving theradiation from the aperture on a collimating mirror and subsequentlyreflecting said radiation; c. impinging the radiation on a prism face;d. dispersing said radiation by passing said radiation through theprism; e. receiving the dispersed radiation from the prism on an echellegrating; f. reflecting said radiation as horizontal and verticalcomponents; g. varying the horizontal and vertical components of theradiation at an exit focal plane by: